There is perceived a need in industry today for a person or sensing device to be able to view the interior of many types of process vessels or containers used in industry. Often, such vessels will contain a corrosive or otherwise hazardous material, and/or its contents will be at elevated pressures and/or temperatures, so that the person wishing to look inside the vessel cannot safely open the vessel for direct viewing.
Sight glasses are commonly used to solve this problem, a sight glass typically comprising a transparent (or at least translucent) component, capable of allowing the viewer to sense the inside of the vessel, and a housing assembly, capable of holding the transparent component in place and attaching to a vessel whose interior it is wished to be viewed. Several varieties of sight glasses are available, each having its own set of limitations. For high pressure applications requiring a sight glass that is hermetically sealed, the prior art sight glasses tend to be made using a variety of methods.
One common method of sight glass manufacture is known as the fusion method. The fusion method includes a transparent component, typically a glass or similar fusible material, formed in the shape of a circular disc, and a metallic ring. The metallic ring is part of the housing assembly and is formed such that the transparent component is fused to the interior diameter of the metallic ring. An example of a sight glass constructed according to a fusion method is disclosed in U.S. Pat. No. 4,472,223 to Bowsky. In order to bond the transparent component to the metallic ring, the two components are heated to relatively high temperatures, traditionally about 800 to 1100 degrees Celsius, to fuse the glass to the metal. At these elevated temperatures, the transparent material becomes soft and flows into direct contact with the metallic material. In the resultant sight glass, the transparent component is bonded both chemically and physically to the metallic ring. Such sight glasses are typically pressure resistant and durable, but they are relatively expensive to manufacture, due to the costs associated with the equipment necessary for achieving the required elevated temperatures. Moreover, the required relatively high temperatures also make it difficult to use certain metallic ring materials, for example brass, which have lower melting point temperatures and/or have the tendency to undergo relatively rapid oxidation at high temperatures.
Under a second known method of sight glass construction, the transparent component is not fused to the metallic ring, but rather is mechanically held in place by one or more gasketing materials. An example of such a sight glass is disclosed in U.S. Pat. No. 4,169,309 to Meginnis. It is well known that such gasketing materials can be configured, along with the transparent component and the other elements of the housing assembly, in such a way that the gasketing material is placed in compression between a surface of the transparent component and a surface of at least one element of the housing assembly. Such configurations can form a sight glass that is semi-hermetic (less than 1×10−5 atms/cc sec) sealed. However, sight glasses made using gasketing materials can be expensive, due to the multiplicity of parts and the labor required for their assembly being mostly of polymeric nature. Moreover, such sight glasses cannot be used in situations where the gasketing materials are incompatible with the temperatures, pressures, and/or corrosive conditions expected inside the process vessel whose interior it is wished to be viewed.
Thus it is an object of this invention to provide a simple, cost-effective method of manufacturing a hermetically-sealed sight glass that avoids the disadvantages of the prior art.